Surface-emission type light-emitting diode and fabrication process therefor

ABSTRACT

An n-type GaAs layer as a buffer layer, an n-type (Al 0 .7 Ga 0 .3) 0 .5 In 0 .5 P layer, an active layer, a p-type (Al 0 .7 Ga 0 .3) 0 .5 In 0 .5 P layer, a thin layer of Al x  Ga 1-x  As layer (x≧0.9), an Al 0 .7 Ga 0 .3 As layer as a current spreading layer and a high doped p-type GaAs cap layer are sequentially grown on an n-type GaAs layer of a substrate. As the active layer, an (Al x  Ga 1-x ) 0 .5 In 0 .5 P based bulk or multi-quantum well is employed. As the current spreading layer, an Al x  Ga 1-x  As (x≧0.7) is employed. The current spreading layer is a p-type III-IV compound semiconductor having wider band gap than a band gap of a material used for forming the active layer, and being established a lattice matching with the lower p-type cladding layer. After mesa etching up to the cladding layer, growth of selective oxide is performed at a part of the AlGaAs layer. BY this, a block layer (selective oxide of AlGaAs) is formed. By this blocking layer, a light output power and a coupling efficiency are improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an optical device to beemployed for optical communication system. More specifically, theinvention relates to a high-brightness visible light-emitting diode(LED) to be employed as high-efficiency light source in a plasticoptical fiber (POF) based optical data-link system. Such LED is alsoapplicable for an outdoor display device, an automotive indicator and soforth.

2. Description of the Related Art

A light output efficiency of the conventional light-emitting diode (LED)is mainly dependent on a structure and shape of an electrode to be usedfor injecting the current to junction. Conventionally fabricatedsurface-emission type light-emitting diode has a quadrangular orcircular-shaped upper electrode located at the center portion thereof.The area and shape of the upper electrode significantly influence thelight output efficiency. For achieving higher light output efficiency,it is required to make the area of the electrode smaller and the area ofa light-emitting surface wider. In the conventional LED, whileemployment of the quadrangular or circular-shaped electrode positionedat the center may facilitate fabrication, but it may cause disruption ofgaussian beam pattern of an output beam. Especially, it may make thebeam having the peak of the emission pattern at the circumferential edgeportion of the electrode. The emission pattern attenuate according toincreasing of distance from the electrode. This is caused by the factthat the current is concentrated at the center portion and notdistributed at the position, distanced from the electrode. A beamintensity to be attained is directly proportional to a current densitythereat. The conventional LED having wide beam pattern as set forthabove cannot be employed in an optical data-link system. That is, evenwhen a large core fiber is employed, the coupling efficiency is held lowdue to emission pattern.

One of typical examples of the conventional surface emission type LED isshown in FIGS. 1A, 1B and 2. FIGS. 1A and 1B show sections along anupper surface of an LED and an approximated near-field-emission. InFIGS. 1A and 1B, an n-type GaAs buffer layer 2, an n-type (Al_(x)Ga_(1-x))In_(1-x) P layer 3, an active layer 4, a p-type (Al_(x)Ga_(1-x))₀.5 In₀.5 P layer 5, a current spreading layer 7 and a p-typeGaAs cap layer 8 are grown sequentially on an n-type GaAs substrate 1.For a p-type contact, quadrangular, circular or cross shaped electrode28 is employed in the prior art. As the current spreading layer 7, athick Al₀.7 Ga₀.3 As layer is normally employed.

In FIG. 2, an approximated near-field-pattern of a light intensity 30for LED having a quadrangular or circular shaped electrode located atthe center, is shown. As shown, the light intensity 30 is maximum aroundthe peripheral edge portion of the electrode and becomes lower atdistance position from the electrode. This represents crowding of thecurrent at the center and not distributing far away for the electrode.In this case, it is impossible to enhance the output efficiency asexpected. In order to avoid crowding of current around the electrode, ablocking layer 31 is typically employed in front of the AlGaAs layer(current spreading layer) 7. FIG. 3 shows another example of theconventional LED having the blocking layer 31. Requirement of regrowthmakes production cost of the LED higher. Also, darkness at the centerportion in the case of circular electrode makes the coupling efficiencylowers even with a large core fiber. It is highly desirable to providean LED structure which can provide high coupling efficiency and issuitable for mass scale production.

III-V semiconductor based visible LED (wavelength is in a range from 580to 670 nm) which has quadrangular/circular shaped electrode at thecenter, have been disclosed in various publications and patentpublications. In every case, quadrangular or circular shaped electrodeprovided at the center is used as upper contact for injecting thecurrent. One of typical examples can be found in the paper of Sugawaraet al., "Japan Journal of Applied Physics, Part 1, Vol. 31, No. 8, pp2446 to 2451, 1992". In this report, a cross shaped electrode along withblocking layer were used in the surface-emission type LED.

However, the light output efficiency is still beyond its practicalapplication. To enhance current spreading outside the contact, a thickwindow layer having low resistivity is required. Even with such thickwindow layer, the current spreading outside the contact is limited to acertain level. Thus, the light intensity is much lower than thatrequired for practical application. However, in application, such asshort distance data-link system, especially based on POF, employment ofthe conventional LED exhibits low coupling efficiency and thus isimpractical in a POF based communication system. For POF based data linkapplication, it would be highly desirable to design the LED which offersnot only high brightness but also high coupling efficiency. In addition,it is highly desirable to reduce a fabrication cost as low as possiblein order to contribute for lowering of a cost for the system.

Japanese Unexamined Patent Publication (Kokai) No. Heisei 2-174272discloses a high brightness LED. In the disclosed art, the brightness ofthe LED is enhanced employing n-p-n-p structure under the contact, whichassists for spreading the current outside the contact. The fabricationprocess of the conventional LED as disclosed in the above mentionedpublication includes a first step of sequentially growing of an N-typejunction layer, a P-type light emitting layer and an N-type transparentlayer on an N-type semiconductor substrate, a second step of performingphoto-etching for the N-type transparent layer, a third step ofperforming diffusion of Zn for forming a current spreading region, afourth step of performing photo-etching for isolation of elements andpolishing of the lower portion of the element, and fifth step of formingelectrode on upper and lower portions of the element. The main drawbackof the disclosed invention is that prior to form p-type contact, a mesastructure reaching to the active region has to be formed in a lightemitting surface to define a current path. Also, a dopant, such as Zn,has to be diffused at high temperature environment. The post-growth hightemperature treatment for Zn diffusion influences on performancecharacteristics due to degradation of the active region. As the lightemitting surface with high p-doping is used, a large fraction of light(depending upon the light energy) can be absorbed in the diffused layer.In fabrication of this type of LED, several times of processes arerequired to cause high fabrication cost of the LED. In addition,difficulty may be encountered in fabrication of high speed LED for highcapacitance caused by wide contact area.

Japanese Unexamined Patent Publication No. Heisei 3-190287 discloses ahigh brightness LED. In the disclosed conventional LED, a plurality oflight emitting regions of mesa structure are arranged on a direction.Each of the light emitting regions is formed so that edges of the lightemitting region perpendicular to the arranging direction thereof becomeforward mesa structure and remaining edges parallel to the arrangingdirection become reverse mesa structure. And an electrode is lead fromonly one of edges having forward mesa structure. In such LED array, forfacilitating wire bonding, the contact was made on the upper layerfollowing the mesa formation. In this case, the shape of the contact isvaried to achieve high light output. The disclosed prior art iscompletely related to the LED array, and since the mesa structure isrequired to be formed prior to the electrode formation, there shouldhave difficulties in formation of the electrode unless a thick contactis formed. This is not only results in increasing of the fabricationcost of the LED, but also impractical for application in fabrication ofsingle LED.

Japanese Unexamined Patent Publication No. Heisei 5-211345 discloses anLED. In the disclosed prior art, between a p-type clad layer and a caplayer, an n-type current blocking layer is provided. In conjunctiontherewith, a portion to be the peripheral edge of the upper electrode inthe current blocking layer is converted into p-type by diffusion or ionimplantation of a p-type impurity through a portion of a lightextraction surfaces on the cap layer where an upper electrode is notmounted. In such prior art, the current blocking layer, the type ofwhich is opposite type (p or n type) of compound semiconductor, anddiffusion at high temperature are used for fabrication of the LED. Inaddition, the configuration of the upper electrode of the LED isimpractical to be employed as a light source in the optical data-linksystem for low coupling efficiency. Furthermore, high temperaturetreatment degrades the optical characteristics due to dopant diffusionand also crystal defect.

The Japanese Unexamined Patent Publication No. Heisei 4-174567 disclosesa high brightness LED array to be employed in a printer. The disclosedconventional surface emission type light emitting diode array has aplurality of surface emission type light emitting diodes extracting alight generated in a plurality of active layers provided on a commonsubstrate, through light extracting surfaces formed at opposite side ofthe substrate. Also, a light reflector layer of semiconductor multiplelayers are provided. The light reflector layers are respectivelyprovided in between active layer of surface emission type light emittingdiode and the common substrate for reflecting the light by light waveinterference. In such prior art, a bottom distributed bragg reflectorlayer (DBR) is used for reflecting the light back from the substrate. Inthis case, the same idea for fabricating the surface emission laser isimplemented. For the DBR, pairs of GaAs/AlAs is used for reflecting a880 nm wavelength light toward the substrate. The types and number ofpairs to be used in the DBR should be dependent on the output emissionwavelength. Also, the same DBR cannot be used in the visible LED where600 to 650 nm wavelength are concerned.

Namely, the disclosed LED array can be used in the printer but cannot beemployed in the data link system. This is because the configuration ofthe upper electrode is he same as that of the conventional LED as setforth above. the coupling efficiency with a fiber becomes quite low assuch LED is employed.

Japanese Unexamined Patent Publication No. Heisei 4-259263 discloses avisible LED employing InAlGaP. The disclosed conventional semiconductorlight emitting element is fabricated by sequentially growing n-type cladlayer formed of InGaAlP type material, active layer and p-type cladlayer, on an n-type GaAs substrate to form a double heterostructureportion. A p-type intermediate band gap layer is formed on the doubleheterostructure portion, and a p-type contact layer is selectivelyformed on the intermediate band gap layer. The semiconductor lightemitting element is consisted of an active layer formed with an orderedlayer having natural super lattice, and the intermediate band gap layer15 formed with a non-ordered layer, in which the natural super latticeis extinguished by Zn diffusion. By making the band gap of theintermediate band gap layer greater than the band gap of the activelayer, the light from the light emitting region can be extracted withoutblocking by the electrode at the light extracting side. In the disclosedprior art, an additional layer of InGaP, band gap wider than the layerused in active region is used in order to avoid Zn diffusion into theactive region. No additional layer for current spreading is used. As setforth above, this type of the visible LED with centrally locatedcircular shaped electrode can not be used in the short distance datalink system.

As explained in the above, the LED structure so far proposed, has lightoutput and coupling efficiency, not enough for using in the shortdistance data link system, especially where POF based system areconcerned. This is because, conventional LED has low output power andalso low coupling efficiency with the POF. Therefore, it would be highlydesirable to design a high brightness visible LED which could befabricated in low cost and suitable for mass scale production.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a surface emissiontype light-emitting diode and production process therefor, which canoffer high output, high coupling efficiency with POF, and highbrightness with uniformly spreading current outside a contact.

The problems set forth above may be alleviated for its potentialapplication in POF or silica based data link system or in any otherdisplay system.

These problems can be alleviated by uniformly distributing current onthe emitting surface, and this can be possible by preventing currentfrom spreading toward bonding region. If current is not allowed tospread toward the bonding region, current could be injected only to thejunction of the emitting surface, circular shaped emitting surface isused to match the outgoing beam shape with the fiber core, resulting inincreased coupling efficiency. Uniform emission pattern is also achievedby optimizing (selection thereof depends on dimension of fiber) theemitting surface diameter, the selection of which also dependent onfiber dimension. The LEDs proposed in the invention can also be usefulin display system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given herebelow and from the accompanying drawings of thepreferred embodiment of the invention, which, however, should not betaken to be limitative to the present invention, but are for explanationand understanding only.

In the drawings:

FIGS. 1A and 1B are schematic diagrams of the conventional visible LED;

FIG. 2 is an illustration showing approximated near field pattern of theLED of FIGS. 1A and 1B;

FIG. 3 is a schematic diagram showing a structure of the conventionalLED;

FIGS. 4A and 4B are illustrations showing the first embodiment of an LEDaccording to the present invention;

FIG. 5 is a graph showing a result of calculation representingdependency of light output efficiency on a dimension of light emittingsurface;

FIG. 6 is an illustration showing the second embodiment of the LEDhaving a selective oxide layer as a DBR and a block layer;

FIGS. 7A and 7B are illustrations showing a DBR structure employed inthe second, fourth, sixth, eighth and twelfth embodiment of the LEDstogether with reflection index characteristics;

FIG. 8 is an illustration showing the third embodiment of the LED havingan ion implantation blocking layer;

FIG. 9 is an illustration showing the fourth embodiment of the LEDhaving the ion implantation block layer and a bottomed DBR;

FIG. 10 is an illustration showing the fifth embodiment of the LEDhaving mesa etched for avoiding spreading of a current at lower side ofthe bonding portion;

FIG. 11 is an illustration showing mesa etched the sixth embodiment ofthe LED having the bottomed DBR;

FIG. 12 is an illustration showing the seventh embodiment of the LEDhaving a blocking layer for improving light output efficiency;

FIG. 13 is an illustration showing the eighth embodiment of the LEDhaving a bottom electrode;

FIG. 14 is an illustration showing the ninth embodiment of the LEDhaving a ring shaped electrode;

FIG. 15 is an illustration showing the tenth embodiment of the LEDhaving the ring shaped electrode and the bottomed DBR;

FIGS. 16A and 16B are illustrations showing a mold structure for theeleventh embodiment of the LED together with POF;

FIG. 17 is a graph showing a result of calculation showing dependenciesof coupling efficiency on lens distance and fiber distance; and

FIG. 18 is a graph showing a result of calculation showing dependenciesof coupling efficiency on lens distance and fiber distance.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the present invention will be discussed indetail with reference to the accompanying drawings. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be obvious,however, to those skilled in the art that the present invention may bepracticed without these specific details. In other instance, well-knownstructures are not shown in detail in order to unnecessary obscure thepresent invention.

FIGS. 4A and 4B are explanatory illustration showing a structure of thefirst embodiment of a visible LED according to the present invention.FIG. 4A is a section and FIG. 4B is a top plan view. As shown, an n-typeGaAs layer 2 as a buffer layer, an n-type (Al₀.7 Ga₀.3)₀.5 In₀.5 P layer3, an active layer 4, a p-type (Al₀.7 Ga₀.3)₀.5 In₀.5 P layer 5, a thinlayer of Al_(x) Ga_(1-x) As layer 6 (x≧0.9), a current spreading layerAl₀.7 Ga₀.3 As layer 7, a high doped p-type GaAs cap layer 8 aresequentially grown on a substrate n-type GaAs layer 1. As the activelayer 4, a bulk or multiquantum well of (Al_(x) Ga_(1-x))₀.5 In₀.5 Pbased material of a desired wavelength can be used. In the preferredembodiment, Al_(x) Ga_(1-x) As with x content of ≧0.7 is used as thecurrent spreading layer 7. However, it also covers all p-type III-Vcompound semiconductor having a band gap wider than that of used asactive layer and also have the lattice matching with its underlyingp-type cladding layer.

Since high doping is necessary in the current spreading layer 7 and thecap layer 8, dopant diffusion into the active layer may deteriorateoptical characteristics of the LED during epitaxial growth. In order toprevent the dopant from diffusing into the active layer, a spacer layer5(1) having a thickness depending on thicknesses of the currentspreading layer 7 and the cap layer 8, is formed following formation ofthe active layer 4. As the spacer layer, the same material as the p-typematerial with low doped used as the p-type clapping layer 6 may be used.

After mesa etching reaching up to p-type cladding layer 5(2), selectiveoxide growth in part of AlGaAs 6(2) is performed. This can be done byheating a sample at a temperature over 400° C. in steam atmosphere whichcan be realized by water bubbling. As the Al contents in AlGaAs is morethan 0.9, selective oxide growth can be done in the layer 6(2). Theselective oxide growth should be selected in such a way that the openingportion (unoxide portion) 6(1) is equal to the diameter of a lightemitting surface 12.

A current is injected to a junction through the opening portion 6(1). Inthe preferred embodiment, a thin layer of AlGaAs 6 is used. However, anytype of III-V compound semiconductor which has crystal matching with thecladding layer 5 and has capability of selective oxidation (e.g. AlAs)may be used for forming the thin layer 6. After epitaxial growth, asilicon nitride or silicon oxide 9 in a thickness greater than or equalto 150 nm is deposited at a temperature lower than or equal to 500° C.Subsequently, an opening window through the dielectric layer 9 for thelight emitting surface 12 is formed. Then, a p-type contact electrode 10(10(1) and 10(2)) is formed using a lift-off process.

In the LED, as a current density is higher in comparison with a currentdensity of the optical device like a laser diode, the contact resistancemainly induced due to the p-type electrode, should be as low as 1 ohmfor long time reliability. Therefore, as the p-type contact, morereliable metallization has to be selected in order to avoid any spikeformation in the GaAs cap layer. For the p-type contact, Au:Zn orTi/Pt/Au or Ni/Zn/Au can be used with much higher reliability incomparison with metallization of the normal kind, such as Cr:Au.

For using the LED in the data link system, the optical couplingefficiency with the fiber is more concerned. Therefore, the emissionsurface should be designed so that a maximum coupling efficiency can beachieved. The coupling efficiency is mainly depend on several factors:the type of the fiber to be used and the emission surface of the LED tobe used. In case of the LED, the light emitting surface should bedesigned in such a way that the more light is concentrated at the centerregion rather than outer side. The coupling efficiency is much moredependent on selection of the emitting surface diameter (namely thediameter of the upper surface). The coupling efficiency can be maximizedby optimizing the diameter of the upper electrode which corresponds thelight emitting surface.

As set forth above, when the conventional electrode which is mainlyprovided at the center portion, is employed, the output light isbroadened at the outer side of the LED to lower the coupling efficiency.If a ring shaped electrode and a current blocking under the bondingregion 16 are employed, the light may be concentrated at the center toachieve higher coupling efficiency in comparison with that using theconventional LED. The design of the circular electrode along with thecircular light emitting surface is also important factor for enhancingthe coupling efficiency and its brightness. In order to enhance theoutput power, the diameter of the electrode (diameter of the lightemitting surface) should be optimized.

FIG. 5 shows a dependency of the light emitting surface diameter on thelight output efficiency as a function of current spreading layer(AlGaAs) thickness. When the thicker current spreading layer isemployed, higher brightness can be attained with a given fixed lightemitting surface area. However, when the light emitting surface iswidened, the light output efficiency becomes closer to that of the LEDhaving thicker current spreading layer. In order to achieve the maximumcoupling efficiency (≧50%) with the fiber, the diameter of the lightemitting surface should be designed in such a way that ratio of thefiber core diameter to the light emitting surface is greater than orequal to 5. It has been found that if the step index (SI) POF fiber withthe core size of 0.98 mm and numerical aperture (NA) of 0.5 are used,the ring diameter (light emitting surface) to be used should be as lowas 100 μm with achieving the coupling efficiency higher than or equal to80%.

After forming the p-type contact, the Au 10(2) is plated on the p-typecontact or partially on the portion where a wire bonding is effected,for facilitating wire bonding. This is followed by covering the lightemitting surface 12 by a passivation layer 13. For the passivation layer13. alumina (Al₂ O₃), silicon oxide (SiO₂) or silicon nitride (SiN_(x))can be used. The passivation layer 13 has to be deposited at a roomtemperature or at a temperature lower than or equal to 200° C. Thepassivation layer 13 prevents the light emitting surface 12 fromabsorbing waster molecular or oxygen from the environment and thus makesdevice reliable for operation over a long period. After formation of thepassivation layer 13, the back side of the substrate is polished toaround 150 μm. Then, the n-type contact 11 is formed thereon. For then-type contact 11, Au:Ge/Ni/Au can be used. After formation of then-type contact 11 on the back side of the polished substrate, each LEDis scrubbed for the packaging. The use of the blocking layer (in thiscase selective AlGaAs oxide) and simple structure of LED is effectivenot only for improving light output power and coupling efficiency, butalso for minimizing fabrication cost of the LED.

FIG. 6 shows an explanatory diagram of the second embodiment of the LED,in which like parts to the foregoing first embodiment will be indicatedby the like reference numerals in the first embodiment forsimplification of disclosure with avoiding redundant discussion. In thesecond embodiment, the distributed bragg reflector (DBR) mirror 14 isgrown on the buffer layer prior to growing of the cladding layer 3, theactive layer 4 and the current spreading layer 7. The DBR to be used asthe mirror 14, consists of the semiconductor pairs of 14(1) and 14(2)having high refraction index difference. Number and the type of thepairs also determine the level of the reflection of the DBR. Thesemiconductor layer to be used as the DBR should also have lowabsorbency for the emitted light.

FIGS. 7A and 7B show explanatory example of the DBR along with itsapproximated reflecting characteristics. As set forth above, number ofpairs as well as their type to be used in the pairs are dependent on theoutput light wavelength. In the optical device, especially the LED, asemission spectra covers the broad wavelength, the DBR to be used shouldhave high reflectivity in broad wavelength range. As shown in FIG. 7B,the maximum reflectivity in broad wavelength, ranging from λ₁ to λ₂should be achieved. 20 This would assist to achieve the maximumreflectivity over the wafer area, while there will be shifting of theDBR reflectivity due to non-uniformity of the thickness, if any. Indesigning LED of 650 nm, AlAs, Al_(x) Ga_(1-x) A (x≧0.45), (Al_(x)Ga_(1-x))₀.5 In₀.5 P (x≧0.45), Ga₀.5 In₀.5 P or GaAs can be used as DBRmaterials. The single DBR or discrete DBR can be used to achieve thebroad wavelength ranges.

Table 1 and table 2 summarize pairs type and the DBR characteristicsusable in designing visible LED of wavelength 650 nm.

                  TABLE 1                                                         ______________________________________                                        Summarizing the DBR type and                                                  its characteristics usable in the visible LED                                                                    Wavelength                                                                    Ranges                                     Pair Type      No. of  Reflectivity                                                                              λ.sub.1 -λ.sub.2             (DBR type)     Pairs   (%)         (nm)                                       ______________________________________                                        AlAs/          ≧20                                                                            99.9        620-685                                    Al.sub.0.5,Ga.sub.0.5 As                                                      Al.sub.0.5 In.sub.0.5 P/                                                                     ≧15                                                                            99.9        630-670                                    (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P                                  AL.sub.0.5 In.sub.0.5 P/                                                                     ≧25                                                                            99.9        620-680                                    Ga.sub.0.5 In.sub.0.5 P                                                       AlAs/          ≧15                                                                            99.9        615-685                                    (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P                                  AlAs/GaAs      ≧15                                                                            90.0        610-685                                    (Al.sub.0.95 Ga.sub.0.05).sub.0.5 In.sub.0.5 P/                                              ≧30                                                                            99          620-680                                    (Al.sub.0.02 Ga.sub.0.98).sub.0.5 In.sub.0.5 P                                Al.sub.0.5 Ga.sub.0.5 As/                                                                    ≧30                                                                            >95         625-675                                    (Al.sub.0.95 Ga.sub.0.05).sub.0.5 In.sub.0.5 P                                Al.sub.0.5 Ga.sub.0.5 As/                                                                    ≧15                                                                            >82         600-700                                    GaAs                                                                          Al.sub.0.5 In.sub.0.5 P/                                                                     ≧15                                                                            >90         610-685                                    GaAs                                                                          (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P/                                                ≧15                                                                            >95         625-685                                    (Al.sub.0.95 Ga.sub.0.05).sub.0.5 In.sub.0.5 P                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Summarizing the DBR type and                                                  its characteristics usable in the visible LED                                                                    Wavelength                                                                    Ranges                                     Pair Type      No. of  Reflectivity                                                                              λ.sub.1 -λ.sub.2             (DBR type)     Pairs   (%)         (nm)                                       ______________________________________                                        AlAs/Al.sub.0.5 Ga.sub.0.5 As +                                                              10 + 10 99.9        610-690                                    Al.sub.0.5 Ga.sub.0.5 As/                                                     (Al.sub.0.95 Ga.sub.0.05).sub.0.5 In.sub.0.5 P                                GaAs/AlAs +     5 + 10 93.0        606-698                                    (Al.sub.0.95 Ga.sub.0.05).sub.0.5 In.sub.0.5 P/                               Ga.sub.0.5 In.sub.0.5 P                                                       ______________________________________                                    

In designing DBR, formation of the graded junction to minimize itsresistance is also important similarly to the reflectioncharacteristics. For this, super lattice or graded junction matchingwith pairs is to be used. After formation of the DBR 14, other layers,such as the n-type cladding layer 4, the active layer 4, p-type claddinglayer 5, the current spreading layer 7 and the p-type GaAs cap layer 8are sequentially grown in the single chamber. Other processes followingthis step are the same as those described in the first embodiment.Therefore, discussed for the subsequent steps is neglected formaintaining the disclosure simple enough for facilitating clearunderstanding of the present invention.

FIG. 8 is an explanatory diagram showing the proposed structure of thethird embodiment of LED, in which the like parts are indicated by thelike reference numerals in the first embodiment so that the repeatedexplanations are omitted. In the third embodiment, mesa etching is notrequired. Before deposition of dielectric layer 9, such as silicon oxideor silicon nitride, proton implantation 15 is performed after coveringthe light emitting surface by the thick resist or any other types ofmaterials which can be etched selectively. The proton implantation isdone to make the high resistive region for avoiding the current fromspreading toward the bonding region 16. The selective oxide layer 6(2)used in the first embodiment is not required in the shown embodiment.However, it is also covers the LED structure where the ion implantationalong with the selective oxide layer is also used. The use of ionimplantation along with the blocking layer can help to enhance lightoutput and coupling efficiency. The processes following to this arealready explained in the first embodiment, so that the repeatedexplanation thereof is omitted.

FIG. 9 is an explanatory diagram showing the proposed structure of thefourth embodiment of the LED according to the present invention. In theshown embodiment, the like parts are represented by the like referencenumerals in the first, second and third embodiments so that the repeatedexplanation can be omitted. In the fourth embodiment, after formation ofthe DBR 14, the n-type cladding layer 3, the active layer 4 and thep-type cladding layer 5 are sequentially grown. The current blockingregion is formed by way of proton implantation 15. The processesfollowing this has been discussed already in the second embodiment. Thediscussion for the following step is neglected for avoiding redundantdiscussion.

FIG. 10 is the explanatory diagram showing the proposed structure of thefifth embodiment of the LED according to the present invention. Itshould be noted that the like parts are represented by like referencenumerals in the first embodiment so that the repeated explanation can beomitted. In the fifth embodiment, the thin layer 6 used in the firstembodiment is not used. In the shown embodiment, the current is injectedto the junction using the mesa structure having a slightly wider areathan that of the light emitting surface. The bonding portion 17 islocated outside the mesa structure. After mesa etching, the dielectriclayer is deposited. Other processes following this step are the same asthose described in the first embodiment. Therefore, discussed for thesubsequent steps is neglected for maintaining the disclosure simpleenough for facilitating clear understanding of the present invention.Owing to simple structure, this LED can be fabricated in low cost andthus is suitable for mass scale production.

FIG. 11 is an explanatory diagram showing the proposed structure of thesixth embodiment of the LED according to the present invention. Itshould be noted that the like parts are represented by like referencenumerals in the first, second and fifth embodiments so that the repeatedexplanation can be omitted. In the sixth embodiment, following theprocess step of growing the DBR 14, the n-type cladding layer 3, theactive ;layer 4, the p-type cladding layer 5, the current spreadinglayer 7 and the cap layer 8 are subsequently grown on the DBR. Otherprocesses following this step are the same as those described in thefirst embodiment. Therefore, discussed for the subsequent steps isneglected for maintaining the disclosure simple enough for facilitatingclear understanding of the present invention.

FIG. 12 shows an explanatory diagram illustrating the proposed structureof the seventh embodiment of the LED according to the present invention.It should be noted that the like parts are represented by like referencenumerals in the first embodiment so that the repeated explanation can beomitted. In the shown embodiment, the thin layer 6 and the mesa etchingarea are required. Also, in the shown embodiment, for fabricating theLED, two steps of epitaxial growth are required. After growing thep-type cladding layer 5, a blocking layer 18 is to be grown andsubsequently, a pattern is formed. The blocking layer 18 assistsprevention of current from concentrating under the bonding region. Asthe blocking layer 18, the same n-type material, used as the claddinglayer 3, may be used. After the patterning of blocking layer 18 isperformed, the current spreading layer 7 and a high dope cap layer ofGaAs 8 for the p-type contact are grown sequentially. Other processesfollowing this step are the same as those described in the firstembodiment. Therefore, discussed for the subsequent steps is neglectedfor maintaining the disclosure simple enough for facilitating clearunderstanding of the present invention. The use of the blocking layer 18assists to inject current into the junction without spreading toward thebonding region and to enhance the light output efficiency.

FIG. 13 is the explanatory diagram illustrating the proposed structureof the eighth embodiment of the LED. It should be noted that the likeparts are represented by like reference numerals in the first, secondand seventh embodiments so that the repeated explanation can be omitted.After formation of the DBR 14, the n-type cladding layer 3, the activelayer 4, the p-type cladding layer 5 and the blocking layer 18 are grownsequentially. Other processes following this step are the same as thosedescribed in the second embodiment. Therefore, discussed for thesubsequent steps is neglected for maintaining the disclosure simpleenough for facilitating clear understanding of the present invention.

FIG. 14 is the explanatory diagram illustrating the proposed structureof the ninth embodiment of the LED according to the present invention.It should be noted that the like parts are represented by like referencenumerals in the first embodiment so that the repeated explanation can beomitted. The thin layer 6, mesa etching, the extra passivation layer 13used in the first embodiment, are not required. In the ninth embodiment,a thin layer of GaAs 19 in a thickness of 100 Å is to be used to formthe p-type contact and also for avoiding the absorption of the emittedlight. In addition, a ring (single or more than single) shaped topelectrode is used to spread current uniformly in the spreading layer.Following to deposition of the dielectric material 9, a ring shapedwindow is opened through the dielectric material. Then, evaporation ofthe Au:Zn, Ni/Zn/Au or Cr:Au is carried out for the p-type contact. Bythe lift off, the ring electrode on the light emitting surface 20 isformed. Subsequently, Cr:Au evaporation on the bonding region 21 alongwith Au plating 22 covering entire electrode portion, to produce theLED. As the dielectric material is used for the purpose of bondingportion and also for passivation layer, it should be selected in suchthat no absorption occurs for the emitted light. For example, for 650 nmLED, silicon oxide or alumina may be used as dielectric material 19.Since only a few process steps are required for fabrication of the LED,the cost for fabrication of the chip can be minimized. It should benoted that while the shown embodiment forms the high resistance regionin the current spreading layer by ion implantation, it may be an oxideblocking layer or an insulative blocking layer.

FIG. 15 is the explanatory diagram showing the mold structure for thetenth embodiment of the LED according to the invention. It should benoted that the like parts are represented by like reference numerals inthe first, second and ninth embodiments so that the repeated explanationcan be omitted. In the tenth embodiment, after formation of the DBR 14,the n-type cladding layer 3, the active layer 4, the p-type claddinglayer, the current spreading layer and a thin GaAs layer are grownsequentially. Other processes following this step are the same as thosedescribed in the second embodiment. Therefore, discussed for thesubsequent steps is neglected for maintaining the disclosure simpleenough for facilitating clear understanding of the present invention.

FIGS. 16A and 16B are the explanatory diagrams illustrating the moldstructure for the eleventh embodiment of the LED according to thepresent invention. In the eleventh embodiment, after bonding the LED ona lead frame 24, molding 25 is performed for increasing the couplingefficiency. A lens 26 is formed during molding. For improving thecoupling efficiency and also keeping the packaging cost as low aspossible, the lens 26 to be used is also made from the same type ofmolding material. In POF 27 based data link application, radius (R) ofthe lens should be ≦0.6 mm and distance of the LED from the center ofthe lens 26(c) should be as low as 0.75 mm. The material to be used formolding should have refractive index n≦1.60, and it should be made at atemperature less than 200° C.

FIGS. 17 and 18 show result of evaluation for the molded lens as shownin FIG. 16. It is found that the use the LED 23 with the mold 25 in thestep index (SI) POF 27 having the numerical aperture (NA) of 0.5 andcore diameter of 0.98 mm, can increase the coupling efficiency more than80%. The position (X₁) of the LED 23 with respect to the lens center26(c), the lens distance Z₁ and fiber distance z₂ play an important roleon the coupling efficiency. In order to achieve the coupling efficiencyhigher than or equal to 90%, the minimum Z₁ should be 0.7 mm.

In the preferred embodiment set forth above, the LED implementable inthe POF based optical system is explained. The Si POF was used forshowing the practicability of the proposed LED 23 as the optical sourcein the short distance data link system. However, it can be also used asan optical source in all types of POF or conventional silica fiber basedsystem. Furthermore, the proposed LED is applicable in the outdoorapplication. The LED structure described in the preferred embodiment isalso applicable for the LED of wavelength ranging from 0.5 to 1.6 μm.

The foregoing description of the preferred embodiments of the presentinvention has been presented for the purpose of illustration anddescription, and is not intended to limit the invention to the preciseform disclosed. It has been chosen and described in order to bestexplain the principle of the invention and these can be utilized invarious embodiments and with various modifications as are suited to theparticular use contemplated.

In the present invention, the visible LED with the blocking layer underhe bonding region exhibited higher output power as compared with theconventional one ever developed. It also exhibited coupling efficiencyover 80% with the POF. The use of circular electrode along with circularlight emitting surface assists for spreading the current uniformlyoutside the contact, making the LED high brightness and increasing thecoupling efficiency higher than the LED ever fabricated.

What is claimed is:
 1. A surface emission type diode comprising:a firstconductivity type buffer layer on a first conductivity type substrate; afirst conductivity type cladding layer; an active layer; a secondconductivity type cladding layer; a second conductivity type thin layer;a second conductivity type current spreading layer; a high doped secondconductivity type cap layer sequentially stacked with said firstconductivity type buffer, said first conductivity type cladding layer,said active layer, said second conductivity type cladding layer, saidsecond conductivity type thin layer and said second conductivity typecurrent spreading layer; a mesa structure etched up to said secondconductivity type cladding layer; a blocking layer formed in said secondconductivity type thin layer by selective oxidation up to a mesa sidewall with maintaining a center portion; and, a molded lens formed of amaterial having a refraction index less than or equal to 1.6, a lensdiameter less than or equal to 0.6 mm, and a distance from said lightemitting surface of said surface emission type diode to a top at thecenter of the lens being less than or equal to 0.75 mm.
 2. A surfaceemission type diode as set forth in claim 1, and including a ring shapedelectrode having one or more electrode rings formed on a light emittingsurface side of said diode, and wherein the light emitting surface ofsaid side other than that covered by said ring electrode is covered withan insulative material having transparency for radiated light.
 3. Asurface emission type diode as set forth in claim 2, wherein saidinsulative material is silicon oxide, silicon nitride or alumina.
 4. Asurface emission type diode as set forth in claim 2, wherein adistributed Bragg reflector (DBR) mirror is formed on said firstconductivity type buffer layer.
 5. A surface emission type diode as setforth in claim 2, and including a distributed Bragg reflector (DBR)mirror comprising twelve or more pairs of AlAs(Al₀.5 Ga₀.5)₀.5 In₀.5 P,twenty pairs of AlAs/Al₀.5 Ga₀.5 As, fifteen pairs of As₀.5 In₀.5P/(As₀.5 Ga₀.5)₀.5 In₀.5 P twenty-five pairs of Al₀.5 In₀.5 P/Ga₀.5In₀.5 P or fifteen pairs of AlAs/GaAs.
 6. A surface emission type diodeas set forth in claim 2, wherein a distance between a step index (SI)POF end surface having numerical aperture (NA) is less than or equal to0.5 and the lens diameter is less than or equal to 1.0 mm in order toobtain a coupling efficiency higher than or equal to 80%.
 7. A surfaceemission type diode as set forth in claim 1, wherein a distributed Braggreflector (DBR) mirror is formed on said first conductivity type bufferlayer.
 8. A surface emission type diode as set forth in claim 7, andincluding a distributed Bragg reflector (DBR) mirror comprising twelveor more pairs of AlAs/(Al₀.5 Ga₀.5)₀.5 In₀.5 P twenty pairs ofAlAs/Al₀.5 Ga₀.5 As, fifteen pairs of As₀.5 In₀.5 P/(As₀.5 Ga₀.5)₀.5In₀.5 P, twenty-five pairs of Al₀.5 In₀.5 P/Ga₀.5 In₀.5 P or fifteenpairs of AlAs/GaAs.
 9. A surface emission type diode as set forth inclaim 7, wherein a distance between a step index (SI) POF end surfacehaving numerical aperture (NA) is less than or equal to 0.5 and the lensdiameter is less than or equal to 1.0 mm in order to obtain a couplingefficiency higher than or equal to 80%.
 10. A surface emission typediode as set forth in claim 1, wherein a diameter of a light emittingsurface is less than or equal to 100 μm.
 11. A surface emission typediode as set forth in claim 1, wherein a distance between a step index(SI) POF end surface having numerical aperture (NA) is less than orequal to 0.5, and the lens diameter is less than or equal to 1.0 mm inorder to obtain a coupling efficiency higher than or equal to 80%.
 12. Asurface emission type diode as set forth in claim 1, and including adistributed Bragg reflector (DBR) mirror comprising twelve or more pairsof AlAs/(Al₀.5 Ga₀.5)₀.5 In₀.5 P, twenty pairs of AlAs/Al₀.5 Ga₀.5 As,fifteen pairs of As₀.5 ₀.5 In₀.5 P/(As₀.5 Ga₀.5)₀.5 In₀.5 P, twenty-fivepairs of Al₀.5 In₀.5 P/Ga₀.5 In₀.5 P or fifteen pairs of AlAs/GaAs. 13.A surface emission type diode comprising:a first conductivity type GaAsbuffer layer; a first conductivity type (Al_(1-x) Ga_(x))₀.5 In₀.5 Pcladding layer; an active layer of bulk or quantum well structure of(Al_(1-x) Ga_(x))₀.5 In₀.5 P; a second conductivity type cladding layer;a second conductivity type thin layer of Al_(x) Ga_(1-x) As (0.9≦X≦1); asecond conductivity type Al_(x) Ga_(1-x) As (0.7≦X≦1) current spreadinglayer; a high doped second conductivity type cap layer sequentiallystacked with said first conductivity type buffer, said firstconductivity type cladding layer, said active layer, said secondconductivity type cladding layer, said second conductivity type thinlayer and said second conductivity type current spreading layer on afirst conductivity type substrate, a mesa structure etched up to saidsecond conductivity type cladding layer; a blocking layer formed in saidsecond conductivity type thin layer by selective oxidation up to a mesaside wall with maintaining a center portion; and, a molded lens formedof a material having a refraction index less than or equal to 1.6, alens diameter less than or equal to 0.6 mm, and a distance from saidlight emitting surface of said surface emission type diode to a top atthe center of the lens being less than or equal to 0.75 mm.
 14. Asurface emission type diode as set forth in claim 13, wherein a diameterof a light emitting surface is less than or equal to 100 μm.
 15. Asurface emission type diode comprising:a first conductivity type bufferlayer on a first conductivity type substrate; a first conductivity typecladding layer; an active layer; a second conductivity type claddinglayer; a second conductivity type thin layer; a second conductivity typecurrent spreading layer; a high doped second conductivity type cap layersequentially stacked with said first conductivity type buffer, saidfirst conductivity type cladding layer, said active layer, said secondconductivity type cladding layer, said second conductivity type thinlayer and said second conductivity type current spreading layer; a highresistance region being formed in said second conductivity type currentspreading layer by way of ion implantation; and a molded lens formed ofa material having a refraction index less than or equal to 1.6, a lensdiameter less than or equal to 0.6 mm, and a distance from said lightemitting surface of said surface emission type diode to a top at thecenter of the lens being less than or equal to 0.75 mm.
 16. A surfaceemission type diode as set forth in claim 15, and including a ringshaped electrode having one or more electrode rings formed on a lightemitting surface side, and the light emitting surface other than saidring electrode is covered with an insulative material havingtransparency for radiated light.
 17. A surface emission type diode asset forth in claim 15, wherein a distributed Bragg reflector (DBR)mirror is formed on said first conductivity type buffer layer.
 18. Asurface emission type diode as set forth in claim 15, and including adistributed Bragg reflector (DBR) mirror comprising twelve or more pairsof AlAs/(Al₀.5 Ga₀.5)₀.5 In₀.5 P, twenty pairs of AlAs/Al₀.5 Ga₀.5 As,fifteen pairs of As₀.5 In₀.5 P/(As₀.5 Ga₀.5)₀.5 In₀.5 P, twenty-fivepairs of Al₀.5 In₀.5 P/Ga₀.5 In₀.5 P or fifteen pairs of AlAs/GaAs. 19.A surface emission type diode as set forth in claim 15, wherein adistance between a step index (SI) POF end surface having numericalaperture (NA) is less than or equal to 0.5, and the lens diameter isless than or equal to 1.0 mm in order to obtain a coupling efficiencyhigher than or equal to 80%.
 20. A surface emission type diodecomprising:a first conductivity type buffer layer on a firstconductivity type substrate; a first conductivity type cladding layer;an active layer; a second conductivity type cladding layer; a secondconductivity type thin layer; a second conductivity type currentspreading layer; a high doped second conductivity type cap layersequentially stacked with said first conductivity type buffer, saidfirst conductivity type cladding layer, said active layer, said secondconductivity type cladding layer, said second conductivity type thinlayer and said second conductivity type current spreading layer; a mesastructure etched up to said second conductivity type cladding layer in adiameter being a size larger than the diameter of a light emittingsurface; and, a molded lens formed of a material having a refractionindex less than or equal to 1.6, a lens diameter less than or equal to0.6 mm, and a distance from said light emitting surface of said surfaceemission type diode to a top at the center of the lens being less thanor equal to 0.75 mm.
 21. A surface emission type diode as set forth inclaim 20, and including a ring shaped electrode having one or moreelectrode rings formed on a light emitting surface side, and the lightemitting surface other than said ring electrode is covered with aninsulative material having transparency for radiated light.
 22. Asurface emission type diode as set forth in claim 20, wherein adistributed Bragg reflector (DBR) mirror is formed on said firstconductivity type buffer layer.
 23. A surface emission type diode as setforth in claim 20, and including a distributed Bragg reflector (DBR)mirror comprising twelve or more pairs of AlAs/(Al₀.5 Ga₀.5)₀.5 In₀.5 P,twenty pairs of AlAs/Al₀.5 Ga₀.5 As, fifteen pairs of As₀.5 In₀.5P/(As₀.5 Ga₀.5)₀.5 In₀.5 P, twenty-five pairs of Al₀.5 In₀.5 P/Ga₀.5In₀.5 P or fifteen pairs of AlAs/GaAs.
 24. A surface emission type diodeas set forth in claim 20, wherein a distance between a step index (SI)POF end surface having numerical aperture (NA) is less than or equal to0.5, and the lens diameter is less than or equal to 1.0 mm in order toobtain a coupling efficiency higher than or equal to 80%.
 25. A surfaceemission type diode comprising:a first conductivity type buffer layer ona first conductivity type substrate; a first conductivity type claddinglayer; an active layer; a second conductivity type cladding layer; asecond conductivity type thin layer; a second conductivity type currentspreading layer; a high doped second conductivity type cap layersequentially stacked with said first conductivity type buffer, saidfirst conductivity type cladding layer, said active layer, said secondconductivity type cladding layer, said second conductivity type thinlayer and said second conductivity type current spreading layer; a ringshaped first conductivity type or insulative blocking layer formed insaid second conductivity type current spreading layer at the side in thevicinity of said second conductivity type cladding layer; said diodefurther including a molded lens formed of a material having a refractionindex less than or equal to 1.6, a lens diameter less than or equal to0.6 mm, and a distance from said light emitting surface of said surfaceemission type diode to a top at the center of the lens being less thanor equal to 0.75 mm.
 26. A surface emission type diode as set forth inclaim 25, and including a ring shaped electrode having one or moreelectrode rings formed on a light emitting surface side, and the lightemitting surface other than said ring electrode is covered with aninsulative material having transparency for radiated light.
 27. Asurface emission type diode as set forth in claim 25, wherein adistributed Bragg reflector (DBR) mirror is formed on said firstconductivity type buffer layer.
 28. A surface emission type diode as setforth in claim 25, and including a distributed Bragg reflector (DBR)mirror comprising twelve or more pairs of AlAs/(Al₀.5 Ga₀.5)₀.5 In₀.5 P,twenty pairs of AlAs/Al₀.5 Ga₀.5 As, fifteen pairs of As₀.5 In₀.5P/(As₀.5 Ga₀.5)₀.5 In.sub.₀.5 P) twenty-five pairs of Al₀.5 In₀.5P/Ga₀.5 In₀.5 P or fifteen pairs of AlAs/GaAs.
 29. A surface emissiontype diode as set forth in claim 25, wherein a distance between a stepindex (SI) POF end surface having numerical aperture (NA) is less thanor equal to 0.5, and the lens diameter is less than or equal to 1.0 mmin order to obtain a coupling efficiency higher than or equal to 80%.